During the past several years, polymeric materials have been developed that possess heat resistance and strength properties found previously only in metals. In addition, the polymers are much lighter than metals, an important advantage where weight is a factor as in modern, high speed aerospace applications. By utilizing structure-property relationships, such as aromatic rings for thermal stability and aromatic-heterocyclic rings for adhesive and cohesive characteristics, it is possible to tailor polymer structures to provide desired end-use properties, such as strength, adhesiveness, elasticity, solvent-resistance, etc. While it may thus be possible to provide a suitable polymer system for a given application, the problem of processing the polymer into an end-use item must also be considered. The processing problem has probably been the most restrictive factor in limiting the use of high temperature resistant polymers.
To process a polymer into a composite structure, it is necessary to cause the polymer to flow in order to impregnate the reinforcing substrate and mold to the desired form. The lower the softening point (Tg) or the melting point (Tm) of a polymer, the easier it is to cause the polymer to flow. In general, a softening point of about 200.degree. C or below is most desirable. While a composite fabricated with a polymer having a softening point of 200.degree. C is suitable for use at 30.degree. C, it will soften and lose its strength at temperatures approaching 200.degree. C. To render the composite suitable for use at temperatures greater than 200.degree. C, a method is required for subsequently raising the softening point of the polymer higher than the desired maximum use temperature. The conventional method of raising polymer softening points is to cure the polymer by joining new chemical bonds or crosslinks between polymer chains. In the curing method most widely employed, a trifunctional monomer is used in the polymer synthesis to provide crosslinking sites along the polymer backbone. This method often leads to branching and gelation during synthesis or storage of prepreg solutions. Other methods for accomplishing crosslinking include radiation, addition of a free radical source, incorporation of a pendant group which can react thermally or chemically, and thermal scission of C--H bonds in the polymer backbone.
There are three major disadvantages to the crosslinking method of cure. One disadvantage results from the evolution of volatiles from any type of cure in which a condensation reaction is used. Because of the volatiles evolution, voids are formed by entrapped gases, effectively weakening the composite structure. A second disadvantage derives from the brittleness which is inherent in a three dimensional network. The third disadvantage lies in the fact that the softening point is raised only as high as the cure temperature because of "freezing in" of the reactive sites when the polymer softening point reaches the cure temperature. In other words, the polymer begins to soften as the use temperature approaches the cure temperature.
It is an object of this invention, therefore, to provide quinoxaline polymers which can be converted to thermally stable, highly fused quinoxaline compositions by non-volatile, intramolecular cyclization.
Another object of the invention is to provide a method for synthesizing quinoxaline polymers.
A further object of the invention is to provide a method of curing quinoxaline polymers that is not subject to disadvantages of crosslinking procedures.
Still another object of the invention is to provide quinoxaline polymers which are soluble in aprotic solvents.
A still further object of the invention is to provide a new monomer for use in the synthesis of quinoxaline polymers.
Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure.